Thermoplastic resin, for example polyester resin laminated-aluminum alloy sheet has been already used for material for a can. However, when the adhesion between a laminated thermoplastic resin layer sheet and the aluminum alloy sheet is insufficient, there is a case where the laminated thermoplastic resin layer peels off from the aluminum alloy sheet during the forming process, or the corrosion of the aluminum alloy sheet is advanced from the portion where the adhesion is insufficient. The adhesion is affected by the formability of aluminum alloy sheet and the properties of the laminated thermoplastic resin layer, and further is largely affected by the condition of the surface of the aluminum alloy sheet. Therefore, in order to improve the adhesion between an aluminum alloy sheet and a thermoplastic- resin layer or coating film, an aluminum alloy sheet has conventionally been subjected to the following surface-treatment.
(1) A method in which an aluminum alloy sheet is subjected to phosphoric acid treatment or chromic acid treatment. PA0 (2) A method in which a thermosetting resin primer is coated on one side of the thermoplastic resin film or aluminum alloy sheet. PA0 (3) A method in which an anodic oxide film is formed on the aluminum alloy sheet by anodic oxidation, the oxide film by anodic oxidation having micro pores with the diameter 2000 Angstrom or more, and the depth 5 .mu.m or less and the occupied area rate by micro pores of 5 to 60% (Japanese Laid-Open Publication No. Hei 3-44496). PA0 (4) A method in which after rinsing of an aluminum alloy sheet, said sheet is heated to the temperature of from 250.degree. C. to 650.degree. C. for two hours or more in the atmosphere to form an oxide film 20 Angstrom or more (Japanese Laid-Open Publication No. Hei 6-272015). PA0 (5) A method in which after rinsing of an aluminum alloy sheet, said sheet is subjected to an electrolytical treatment using alternating current in an alkali solution to form an oxide film having a thickness of 500 to 5000 Angstrom and having branched micro pores (Japanese Laid-Open Publication No. Hei 6-267638).
However, in the above-mentioned method (1) of phosphoric acid treatment or chromic acid treatment, since phosphate, chromate, fluoric compound or others is mainly used as the chemical treatment solution to improve the paint adhesion, the treatment is widely used. However, it requires an enormous draining equipment for the draining of treatment solution in order to prevent the environmental pollution. Therefore, method (1) is undesirable from the aspect of preservation of the environment. Further, in the above-mentioned method (2) of coating with the adhesive primer, cost is increased by primer coating, and besides the process for thermosetting of coating film is additionally needed, so it is undesirable from the aspect of productivity. Further, the equipment for exhausting organic solvent is needed. As for the above-mentioned method (3) of forming an oxide film by anodic oxidation having specific micro pores by using a chromic acid solution, a long time is required for forming the oxide film by anodic oxidation, so it is undesirable from the aspect of productivity. Further, when heavy forming process is carried out, the laminated thermoplastic resin layer occasionally peels off from the aluminum alloy sheet. Furthermore, a draining equipment is needed for the prevention of the environmental pollution. In the above-mentioned method (4) of forming an oxide film by heating the cleaned aluminum alloy sheet for a long time in the atmosphere, when a heavy forming process is carried out, the laminated thermoplastic resin layer peels off from the aluminum alloy sheet as in method (3), and a long time is taken for forming of an oxide film, so it is undesirable from the aspect of productivity. Further, in the above-mentioned method (5) of forming, an oxide film of 500 to 5000 Angstrom by electrolyzing an aluminum alloy sheet by using alternating current in alkali solution, the surface treatment of the aluminum alloy sheet can be continuously carried out by the electrolysis in a short amount of time, and the effective adhesion after forming of the laminated resin film can be obtained. However, after the resin film is laminated on the treated aluminum alloy sheet, when said sheet is subjected to a drawing forming process, followed by a stretching process, further followed by an ironing process, the laminated resin film is peels off from the aluminum alloy sheet. Therefore, it is hard to say that the adhesion after forming is sufficient and the resin-coated aluminum alloy sheet cannot withstand severe forming process.
The objective of the present invention aims to achieve is to provide a process and apparatus for producing a thermoplastic resin-coated aluminum alloy sheet having excellent adhesion from which the laminated thermoplastic resin layer does not peel off even when being subjected to a severe forming process, as compared with a conventional surface-treated aluminum alloy sheet as mentioned herein above, and further from which the laminated thermoplastic resin layer does not peel off even when it is subjected to high temperature steam-treatment (retort treatment) of 100.degree. C. to 300.degree. C. Concretely, the present invention aims at providing an aluminum alloy sheet having excellent adhesion after forming in which the laminated thermoplastic resin layer does not peel off even when the sheet is subjected to a drawing forming process, followed by a stretch-forming process, and further followed by an ironing process, at a low cost. Further the present invention aims at providing a process and an apparatus for producing the same which enable high speed production and are environmental friendly.